The present invention relates to ceramic matrix composites, and more particularly to fiber-reinforced ceramic matrix composites comprising short reinforcing fibers, wherein the alignment of the fibers can be controlled to control the physical properties of the composite product.
At the present time, many glass, glass-ceramic, and ceramic matrix composites are formed from unconsolidated material known as prepreg. Prepreg consists of an assemblage of reinforcing fibers which has been impregnated with a selected matrix material in particulate form, typically as a glass or crystalline ceramic powder. Also present in the prepreg is a binder, usually an organic polymer, which holds the powdered ceramic material and fibers together.
U.S. Pat. No. 4,615,987 discloses conventional methods for the manufacture of ceramic matrix composites, and describes the preparation of individual tow prepreg and sheet prepreg. Sheet prepreg as currently produced contains continuous fiber tows in three forms: as parallel tows, as parallel tows in combination with individual whiskers (known as "hybrid" prepreg), or as two-dimensionally woven or braided cloths.
Prepreg in the form of simple sheet is typically formed by drum-winding continuous impregnated fiber tow or yarn. Prepreg tape can be made by collimating and consolidating multiple continuous tows into a continuous ribbon.
Since prepreg is typically formed of long or continuous fibers or fiber tows which are directionally aligned, either along a single axis or along a few selected axes in a plane, the ceramic composite products formed from prepreg tow inherently have anisotropic (directionally dependent) properties. Uniaxial fiber alignment (all fibers aligned in a common direction) produces composite bars or beams with a single strong axis, while planar fiber arrays (fibers lying flat but in random directions within a flat or curved plane) produce curved or flat sheet with high stiffness and strength in the plane of the fibers.
Composites of complex three-dimensional shape, when made by casting or molding processes, typically comprise shorter, randomly oriented reinforcing fibers and have no preferred strong axis or plane. In many cases the randomly oriented reinforcing fiber phase, and the resulting isotropic strength properties, are deemed desirable. However, this is not always the case.
In U.S. Pat. No. 4,921,518, a process for making a short-fiber composite is described wherein long reinforcing fibers are combined with a glass matrix material, heated to soften the glass, and then chopped to form a partially consolidated pre-product which is shaped by hot glass forming methods into a composite product. In another approach, the combined fiber and matrix material are first chopped to form an unconsolidated glass/short-fiber mixture, and the mixture thereafter subjected to binder burnout and hot-pressing to form the final product.
U.S. Pat. No. 4,511,663 describes a method for ceramic matrix composite manufacture wherein metal-coated fibers are combined with powdered glass to form a prepreg which is cut into pieces, stacked in a mold, and hot-pressed to a final product. In U.S. Pat. No. 4,780,432, composites are made by a glass injection molding process at high temperatures and pressures.
In the field of polymers, it is known to use shortened glass and carbon fibers to reinforce resin matrix materials. U.S. Pat. Nos. 4,856,146, 4,856,147, and 4,857,385, for example, describe the production of so-called stretch-broken fiber yarn for composite reinforcement. The process involves first coating the yarn with a viscous coating, then stretch-breaking individual fibers, and finally incorporating the coated, stretch-broken yarn into the matrix polymer. Unfortunately, viscous-coating and then stretch-breaking fibers is incompatible with conventional ceramic composite fabrication technology, which requires the removal of all sizing and other organics from the fibers prior to coating or infiltration with ceramic matrix materials.
Whereas the use of shorter fibers for complex ceramic shape-forming is advantageous from a processing standpoint, short fibers do not always provide the physical properties required in the final product. Thus, as previously suggested, it is difficult to achieve fiber alignment and high strength along a preselected axis or plane in a complex shape made from conventional short-fiber prepreg materials, since fiber orientation in the latter is typically random. Yet there are many instances where the presence of a strong axis or plane in a complex shape would be of considerable value.
The difficulties in incorporating long, directionally aligned fiber reinforcement in ceramic matrix composites of complex shape are substantial. Conventionally, the volume proportion of void space in ceramic matrix composite preforms following the burnout of all organic binder constituents is in the range of 75 to 80 volume percent. This means that the burned-out preform must debulk by a factor of 3 to 5 during consolidation, in order that a dense composite can be obtained.
Debulking is not a problem when the composites being consolidated have simple flat plate geometries requiring little fiber movement in directions parallel to the fibers. With complex shapes, however, and where long, continuous fibers are present, debulking requires a realignment of preform structure through ply slippage. This interply motion results in either fiber buckling in compression, or fiber breakage in tension, in order that full consolidation can be achieved. And, where the fibers are sufficiently strong that they cannot be broken or buckled, significant void retention in the composite product can be expected. In either case, a weakened product results.